The present invention relates to methods and apparatus for imaging two-dimensional (2-D) planes or surfaces and/or three-dimensional (3-D) objects. The invention is particularly useful in angiography for imaging a person""s vascular system in 2-D or 3-D, and is therefore described below with respect to this application.
Electromagnetic radiation has long been used to produce images of internal structures of the human body for purposes of diagnosis or treatment of diseases. One technique which has gained widespread use since the 1970""s is computerized tomography (CT), sometimes called computerized axial tomography (CAT), in which a narrow beam of X-rays is swept across an area of the body and is recorded by a radiation detector as a pattern of electrical impulses, while a computer is used to integrate the data for many such sweeps and to reconstruct a two-dimensional or a three-dimensional image of the examined volume. In the current CT systems, the radiation source generally produces a fan-shaped beam, and the radiation detector generally includes a line of detector elements aligned with the fan-shaped beam so as to detect the level of the radiation after having traversed the object being examined.
When a 3-D image is to be produced, the body is exposed to the radiation at a plurality of different image planes (slices), each at a different angular position, while a large number of gray levels of radiation are sensed by each detector element. For example, a typical procedure may involve in the order of 256 slices, each at 128 different angular positions, with each detector element recording up to 128, 256, 512 or 1024 gray levels. Many reconstruction algorithms are known for reconstructing the three-dimensional image from this data.
Angiography, on the other hand, involves the radiographic examination of arteries and veins, in which a contrast medium is injected into the vascular system to cause a denser shadow than would be caused by other tissues, thereby enabling the blood vessels to be distinguished from the other tissues. In digital subtraction angiography (DSA), an X-ray image (called a xe2x80x9cmasking imagexe2x80x9d) of the patient is taken before the contrast material is injected; and another X-ray image (called a xe2x80x9ccontrast imagexe2x80x9d) is taken after the contrast material is injected. The masking image is subtracted from the contrast image to leave only the DSA image of the blood vessels, enabling them to be readily distinguished from the other tissue.
According to the prior art, angiographically imaging a portion of a patient""s vascular system thus involves: producing a first sequence of masking images of the portion of the patient""s vascular system; injecting a contrast material into the patient""s vascular system; producing a corresponding sequence of contrast images of the portion of the patient""s vascular system while containing the contrast material and subtracting from each contrast image of the sequence the corresponding masking image in the sequence to thereby produce a difference image having an enhanced contrast-to-noise ratio (CNR).
Examples of known DSA systems are described in U.S. Pat. Nos. 5,630,414, 6,052,476, 6,075,836 and 6,118,845, the disclosures of which are incorporated herein by reference.
Such a subtraction process was generally performed, according to the prior art, by using an ECG sensor or a respiration sensor as the reference point for the subtraction process. However, using a respiration sensor as the reference is not precise because of the difficulty in sharply defining a particular point of the respiration curve for this purpose. Similarly, utilizing an ECG sensor is also not precise for the same reason. Moreover, the mere presence of an ECG sensor can effect the ECG signal detected from the patient""s body.
The value of the images produced by DSA for diagnostic or treatment purposes depends to a high degree on the contrast-to-noise ratio (CNR) of the DSA image. The CNR of an image is to be distinguished from the signal-to-noise ratio (SNR), and is generally defined as being the difference in the SNRs of adjacent imaged regions. Thus, enhancing the CNR of a DSA image would better enable the physician to discern details of the patient""s vascular system and therefore better enable the physician to utilize this information for diagnostic or treatment purposes.
An object of the present invention is to provide an improved method and apparatus for producing two-dimensional or three-dimensional angiographic images to better enable diagnosis or treatment of a disease in the human body. A further object of the invention is to provide a method and apparatus for enhancing the CNR of a DSA image.
According to one aspect of the present invention, there is provided a method of angiographically imaging a portion of a patient""s vascular system, comprising the steps:
(a) producing a first sequence of masking images of the portion of the patient""s vascular system;
(b) determining the masking image in the sequence having a minimum change over its immediately preceding masking image in the sequence;
(c) injecting a contrast material into the patient""s vascular system;
(d) producing a corresponding sequence of contrast images of the portion of the patient""s vascular system while containing the contrast material and
(e) subtracting from each contrast image of the sequence the corresponding masking image in the sequence, starting with the minimum change masking image, to thereby produce a difference image having an enhanced contrast-to-noise ratio (CNR).
It has been found that using the masking image of minimum change as a reference for the digital subtraction operation, instead of an ECG signal or respiration signal, not only obviates the need for an ECG sensor or respiration sensor but, even more importantly, produces a DSA having an enhanced CNR as compared to previous techniques using such sensors. Thus, using the masking image of minimum change as a reference permits more precise time-correlation of the masking images to be subtracted from the corresponding contrast images. Moreover, this technique does not require a special sensor, such as an ECG sensor, which might affect the sensor signal merely because of the presence of the sensor.
According to a further feature in the described preferred embodiment, in the subtraction step, the respective contrast image is multiplied by a factor (k) of 1 to 3 before the corresponding masking image is subtracted therefrom. Preferably, the factor (k) is 1.8. That is, if each masking image in the first sequence (before injection) is subtracted from 1.8 times the corresponding contrast image, starting with the minimum change masking image, a substantial enhancement of the resulting difference image is obtained.
In step (b) of the preferred embodiments described below, a pixel-by-pixel comparison of the gray-level of the pixels in the respective masking image is made with the corresponding pixels in the preceding masking image of the sequence, and the masking image of the sequence having minimum changes over its immediately preceding masking image in the sequence is selected as the masking image of minimum change.
In step (e) of one described preferred embodiment, there is subtracted from each contrast image of the contrast image sequence, the gray-levels of the corresponding masking image in the masking image sequence, starting with the minimum change masking image, to thereby produce a difference image having an enhanced contrast-to-noise ratio (CNR). In step (e) of a second described embodiment, there is subtracted from each contrast image of the contrast image sequence, the gray-levels of the corresponding masking image in the masking image sequence, averaged with the gray-levels of the next masking image in the masking image sequences starting with the minimum change masking image, to thereby produce a difference image having an enhanced contrast-to-noise ratio (CNR). This averaging technique tends to average-out errors in the time-correlation during the subtraction step, and thereby, in many cases, produces a further enhancement of the resulting difference image.
In step (e) of both described embodiments, a pixel-by-pixel comparison of the gray-level of each pixel in the respective contrast image is made with the corresponding pixel in the preceding contrast image of the sequence. The contrast image of the sequence having minimum changes over its immediately preceding contrast image in the sequence is used as the starting point, together with the minimum change masking image, for subtracting from each contrast image the corresponding masking image.
According to still further features in another described preferred embodiment, the first sequence of masking images and the corresponding sequence of contrast images are produced by a radiation source and a radiation detector located on opposite sides of the patient; relative linear movement is effected between the patient and the radiation source and radiation detector during the two image sequences; and a linear positional transformation is performed to normalize each of the images with respect to the relative movement before the masking images are subtracted from the contrast images to produce the difference images having an enhanced CNR.
The linear positional transformation in the described preferred embodiment is performed by measuring the relative change in position of the patient with respect to the radiation source and radiation detector when producing the two sequences of images; measuring the gray-level in each of the pixel positions in each of the masking images and contrast images; and modifying the pixel positions in each of the masking images and contrast images according to the measured relative change in the position of the patient for the respective masking image and contrast image.
The latter feature enables a larger number of the images to be used, and/or a larger region to be examined, in producing the two-dimensional angiograms of enhanced CNR, since it allows use of the angiograms produced while relative movement is effected between the patient and the radiation source and radiation detector, as well as those in which the patient is non-moving.
According to another feature in a described preferred embodiment, the radiation source and radiation detector are carried on a gantry which is rotated around the patient to a plurality of angular positions to produce a sequence of the masking images and the contrast images for each angular position; and a difference image having an enhanced CNR is produced for each of the angular positions. The difference images for the plurality of angular positions may then be reconstructed to produce a 3-D image of enhanced CNR according to presently-known techniques.
According to another aspect of the present invention, there is provided apparatus producing angiographical images of a portion of a patient""s vascular system, comprising: a radiation source to be located at one side of the patient for radiating the portion of the patient""s vascular system; a radiation detector to be located at the opposite side of the patient for producing electrical outputs corresponding to the magnitude of the radiation received by the detector; a computer for controlling the radiation source and for processing the outputs of the radiation detector to produce an image of the portion of the patient""s vascular system; and a display for displaying the produced image; the computer controlling the radiation source and processing the outputs of the radiation detector such that: before a contrast material is injected into the patient""s vascular system and a sequence of masking images is produced of the portion of the patient""s vascular system, a determination is made as to the masking image in the sequence having a minimum change over its immediately preceding masking image; and after a contrast material is injected into the patient""s vascular system and a corresponding sequence of contrast images is produced, from each contrast image the corresponding masking image in the sequence is subtracted starting with the masking image determined to have a minimum change over its immediately preceding masking image, to thereby produce a difference image having an enhanced contrast-to-noise ratio (CNR) which is displayed in the display.
As will be described more particularly below, such method and apparatus can be used for both two-dimensional angiography as well as for three-dimensional angiography.
Further features and advantages of the invention will be apparent from the description below.